Free-machining stainless steel and method



FREE-MACHINING STAINLESS STEEL AND METHOD William Charles Clarke, Jr.,Baltimore, Md., assignor to Armco Steel Corporation, a corporation ofOhio Application December 3, 1951, Serial No. 259,587

4 Claims. (Cl. 75-125) My invention relates generally to chromium-nickelstainless steels, and more particularly concerns the pro duction ofstainless steels displaying free-machining qualities.

An object of my invention is to provide anaustenitic chromium-nickelstainless steel which not only possesses all the advantageous propertiescharacteristic thereof,

but at the same time displays vastly improved machining qualities,responding favorably to standard tests, including tool life tests anddrill tests, which qualities are imparted to the steel at minimum cost,in ready and simple manner.

Yet another object is to provide an austenitic chromium-nickel stainlesssteel displaying exceptional freecutting tools, more time for usefulwork on the part of the operator, and longer tool life, all with reducedinvestment in tools and in sharpening equipment.

Other objects and advantages in part will be obvious and in part pointedout hereinafter during the course of the following description, taken inthe light of the accompanying drawings.

Accordingly, my invention may be seen to reside in the composition ofingredients and combination of ele- United States Patent 2,697,035Patented Dec. 14, 1954 ICC erated at such surface speeds that thecutting tools will retain a proper cutting edge throughout a five hourlife period, and will not require refinishing or regrinding until thecompletion of the work shift.

For a particular metal upon which the cutting operation is to beperformed, therefore, it is essential that a surface speed, or cuttingspeed, be selected for that particular metal such that the wear of thecutting tool 7 will be at such rate that the tool can be kepteffectively ments, and in the art of producing the same, as well as inthe combination of steps employed, the scope of the application of allof which is more fully set forth'in th claims at the end of thisspecification.

Inthe drawing, I disclose a chart wherein the cutting speed in surfacefeet per minute is plotted against the tool life in minutes for thevarious types and grades of metal undergoing cutting tests.

As conducive to a more thorough understanding of certain features of myinvention, it may be noted at this time that many of the highlyadvantageous qualities of chrome-nickel stainless steel alloys cannotpresently be availed of, at least not readily, due to the high rate atwhich these steels work-harden during processing. Tool wear at high rateis observed, and necessity arises for frequent regrinding the tool orreplacement.

Both the duration of processing is prolonged, with attendant appreciablyincreased labor costs, and plant investment is increased. Particularlyis the use curtailed where appreciable machining is required. Rapid toolwear is observed, and the metals must be operated at low surface speeds.

Now for economic utilization of labor, particularly at the high wagestandards prevailing today, it has long been recognized that toolsshould be reground and resharpened only once for each working shift,this at the beginning of the shift. Again, it is found that the totaltime during which the tools are actually in use on an eight-hour shiftaverages about five hours. That means, of course, that given tools ofstandard composition, then for best results the metal to be machinedmust be opits composition.

in use throughout the five hour cutting period. As a matter of fact,these standards are employed in measuring free-machining qualities instandard testing practice, known as the five hour form tool life.

Another measure of the machining qualities of a metal is .the timerequired to drill through a test specimen of the metal of a standardthickness with a test drill of standard composition and standarddiameter under standard constant loading.

Heretofore it has been observed that the austenitic chromium-nickelstainless steels, with their high workhardening rate, can be used onlyinfrequently where subjected to considerable machining. Accordingly,much etfort has been expended towards improving the freemachiningqualities of these metals, while retaining the characteristic advantagesthereof, including smooth lustrous finish, resistance to corrosion,non-magnetic qualities and the like.

It heretofore has been suggested to include an appreciable quantity ofsulfur in the metal, thereby improving, in well-known manner, thechipping qualities of the metal, while also reducing somewhat theabrasive qualities of the metal against the tools. Expressed in otherterms, under cold-operating conditions, some improvement in machiningqualities is observed, as well as an improvement in drilling time, whensulfur is added to the stainless steels. Where desired, the sulfur maybe substituted in whole or in part by selenium.

An important object of my invention, therefore, is to remove or diminishin appreciable respect the many disadvantages of prior practices, and atthe same time produce an austenitic chromium-nickel stainless steeldisplaying free-machining qualities, which steel can be produced inready, simple and direct manner, and which once produced, can bemachined with minimum wear on the cutting tools, with high cuttingspeeds, and withimproved rate of drilling and with the formation ofsmall chips, the use of steel permitting more ready and completeutilization of the operator-s time during the standard work shift.

Referring nowto the practice of my invention, I find that a reallynoteworthy and marked improvement in machining qualities of theaustenitic chromiumnickel stainless steels is achieved, with fullretention of the usual advantageous characteristics of the steel, whenan appreciable amount of .copper is included in I prefer to includecopper, preferably along with one or more of the ingredients sulfur,selenium, tellurium, bismuth, lead and silver in small amounts, theseconveniently being referred to hereinafter as free-machining elements.Preferably I include either sulfur or selenium, or both, in broadanalysis, substantially as follows: chromium 12% to 20%, nickel 6.5% to15%, copper 1.5% to 5%, sulfur .1% to .5%, and/or selenium .l% to .4%,manganese up to 4%, carbon up to .15 and remainder iron. Where asubstantial amount of manganese is used, it is employed in a range of1.75% to 3.75%. If desired, nitrogen may be added up to .15 in order toimpart improved surface. So also phosphorus up to 50% likewise may beadded to improve the surface.

Now, as to the broad composition range, I find that the addition ofcopper drastically reduces the work hardening rate of the alloy.Moreover, I find that when manganese is present some improvement inmachinability is had, particularly in the matter of tool life.

In furtherance of the practice of my invention, I carried out a numberof tests on samples of my steel to cutting the steels.

J ascertain its machinability. The results are shown in the followingtable:

Table Hr. Heat Drill Test No. C or N1 S Se on {32 Time, Sees.

s. F. it

9.00 115 1 N. (1.; 190 9.14 155 9. 05 112 13.11 20s 18 9. 00 12s 2 22 9.09 19s 2 14 9.00 145 8.08 198 1 2s 1s. 97 23s 1 drill. z drill.

N. G. Drill would not penetrate.

These tests, generally as pointed out hereinbefore, are premised on theproposition that given the use of standard cutting tools a commonmeasure of the machinabllity of those steels is the rate at which thesetools wear while In this connection it is to be noted, as alreadymentioned, that in machine shop practice it usually is desirable togrind or resharpen tools only once each shift, and this at the beginningof the shift. This means, in turn, that a tool is in actual cutting useon an average of five hours per shift, and the cutting speed which givesthis tool life is used to express the machinability of a material.

Thus in the table, one test is the so-called five hour form tool lifeexpressing the cutting speed in surface feet per minute.

Another test of the machining or free-cutting qualities of the metal isshown in the so-called drill test. In this test the number of secondsare measured which are required for a drill of given composition andtemper, of fixed size, and under steady load conditions, to penetratethe metal to a certain distance. The more free-cutting is the steelundergoing test, the more rapid is the penetration of the drill into thetest specimen. The drill test measures the ease with which chips areparted from the steel, or the amount of power required to cut it. It isbecause sulfur, and selenium as well, causes the chips to break offreadily, without curling, that some improvement in drilling qualitiesare observed when sulfur or selenium are added to the melt.

It is to be noted that in the form tool life test, my practice is toemploy a No. 4 Warner & Swasey turret lathe, using as a cutting fluid asulfur base oil, Tycol 655, mixed with an equal amount of paraffin baseoil. The forming tool bits are of a standard 18-4-1 type tool steel, andeach tool bit is finished to a smooth surface with a standard wheel in astandard grinder. After failure, the tool is reground Well below anyevidence of burned metal.

The tests are carried out on a one-inch diameter bar of the metalundergoing test, and comprises taking a forming cut one-sixteenth of aninch wide at a feed of .0025 inch per revolution from the surface to thecenter of the bar, the tool being sharpened with a top rake of 8degrees, a front clearance of 7 degrees, and a side clearance of 2degrees. All forming cuts are made close to the collet of the machine.And with the machine speed adjusted to a predetermined value the tool isrequired to make successive cuts until tool failure occurs. This failureis determined by observing the burning and destructive abrasion of thenose of the tool.

In the drill penetration tests, there is required the use of a highspeed inch twist drill or inch twist drill, together with a drill press,providing a spindle speed of 900 R. P. M., equipped to have a constantthrust of 90 pounds on the drilling spindle, and a method of measuringthe depth of penetration of the other drill. The drill is sharpened topenetrate one-half inch of standard bar stock in sixteen seconds. Thestandard used in the laboratory is R12FM Heat 64376 analyzing, carbon.094%, manganese .44%, phosphorus .020%, sulfur .333%,'silicon .32%,chromium 12.21% and nickel .46% tempered to a hardness of 207 Brinell.Under these stand- .ard conditions samples are drilled, noting the timere- 4 quired to drill a one-half inch hole or a one-quarter inch hole,as the case may be.

In studying the test results set forth in the table, it will be observed(Heat 1) that an austenitic l88 chromium-nickel stainless steel with nocopper and with only low sulfur content, the steel has a maximum cuttingspeed for five hour tool life of but surface feet per minute. Moreover,this steel could not be penetrated by a inch drill loaded at 90 pounds,while a full 90 seconds was required for penetration with a inch drill.Of course, the inch drill tests are not quite as determinative, for thedifference is less marked between test specimens with the use of thesmaller drill, this latter not being as sensitive to variations in powerrequirements for chip removal.

With a substantial sulfur content (Heat 2) then with this 18-8chromium-nickel stainless steel of low carbon content, the cutting speedis increased to 155 S. F. M., an improvement of about roughly 30%. FromHeat 3, employing Within practical limits, the same quantity of sulfuras in Heat 2, 112 seconds are required for penetration of a A1 inch testspecimen with a inch drill.

If to the austenitic chromium-nickel stainless steel there be added morenickel, and copper in the amount of 2.99% (Heat 4), the cutting speed isincreased to 205 S. F. M. for five hour tool life, while the timerequired for penetration of the 7 inch drill is reduced to 18 seconds.And with the nickel content high and the chromium content low (Heat 9)and about 3% copper and .3% sulfur, the cutting speed is even greater,namely, 238 S. F. M.

Similarly, with the 18-8 chromium-nickel steel with but 0.158% sulfurand no copper (Heat 5), the maxirnum cutting speed is S. F. M. on fivehour life test, while 22 seconds are required for penetration by theinch drill. And that a like steel with 26% selenium substituted forsulfur and without copper (Heat 7) the cutting speed is S. F. M., forfive hour tool life.

With the copper addition of 3.14% (Heat 6) the cutting speed becomes 196surface feet per minute and the time for drill penetration 14 seconds.And upon the addition of 2.85% copper to the steel with 21% selenium(Heat 8) the cutting speed is 198 surface feet per minute and the timefor drill penetration 23 seconds.

It is apparent from the foregoing that radically improved results attendupon the addition of copper. Cornparing the copper-bearing steels ofHeats 4, 6, and 9 with the standard sulfur and selenium-bearingfreemachining steels of Heats 2, 5, and 7, the average improvement comesto about 35%. And comparing my steel with the usual austeniticchromium-nickel steel (Heat 1) the improvement in machinability amountsto about 75 Further comparison of my steel With prior steels isgraphically illustrated in the accompanying drawing. In this connectionit is to be noted that when the logarithm of the cutting speed insurface feet per minute is plotted against the logarithm of the toollife in minutes, a straight line relationship is obtained. These dataare extrapolated to five hour tool life, which figure is then used inthe comparisons of the test stock. In my tests, upon extrapolating to300 minutes or five hours, it will be seen that with the A group,involving Heat 1, and with no sulfur, selenium or copper, the cuttingspeed is about 115 surface feet per minute. With the B group,constituting the Heats 2 and 7, wherein sulfur and selenium,respectively, are the only additives, the average cutting speed issurface feet per minute. While finally, for the C group, comprisingHeats 4 and 8, wherein copper is added along with sulfur and selenium,respectively, the average cutting speed is slightly more than 200.

From the foregoing it is seen that given constant cutting speed, thetool life is greatly prolonged as a result of the addition of copper,and conversely, for given tool life, the cutting speed is appreciablyincreased. It is apparent, therefore, that by the addition of copper Ihave greatly increased the machinahility of the austeniticchromium-nickel stainless steel. Moreover, longer tool life is observed,with less grinding or sharpening and with more time employed on usefulwork. A reduced investment in tools is required, and a smaller amount ofgrinding and resharpening equipment is employed, while such ficf uipmentas is provided is observed to display longer Moreover, it is to beobserved that the same phenomenom carries over to the drill test, andthe more freecutting be the steel, the more rapid is the penetration ofthe drill. This is due to he decrease in work-hardening and to the easewith which the chips are parted from the metal stock. With the ordinary18-8 chromiumnickel steels, the inch drill will not penetrate the steelat all under a 90 pound load, while the inclusion of sulfur alone, a 4inch drill will penetrate in 112 seconds. With sulfur and 3% copper, thesame hole is drilled in 18 seconds evidencing the reduction inwork-hardening. The same trend is observed when using a 4 inch drill.

Thus in accordance with the practice of my invention I provide achromium-nickel stainless steel displaying ex cellent free-machiningqualities. With this steel improved tool life is had. This result,consistently obtained, ensures, vastly increased unit output. All these,as well as many other highly practical advantages, attend upon thepractice of my invention.

It will be seen from the foregoing that once the broad aspects of myinvention are disclosed, many embodiments thereof will readily suggestthemselves to those skilled in the art, and as well, many modificationsof the disclosed embodiment will likewise come to mind, all fallingwithin the scope of my invention. Accordingly, I intend the foregoingdescription to be considered as merely illustrative, and not bylimitation.

I claim as my invention:

1. Chromium-nickel stainless steels displaying freemachining qualities,together with reduced tool abrasion and free-chipping qualitiescomprising about 18% chromium, 8% nickel, 3% copper, a material selectedfrom the group consisting of about 3% sulfur or .2% selenium, and theremainder iron.

2. Free-machining chromium-nickel stainless steels comprising about 18%chromium, 13% nickel, 3% copper, .2% to .5 sulfur, up to .15 carbon, andremainder iron.

3. Free-machining chromium-nickel stainless steels comprising about 13%chromium, 14% nickel, 3% copper, .3% sulfur, up to .15 carbon, andremainder iron.

4. A free-machining chromium-nickel stainless steel of improved surfacefinish comprising 12% to 20% chromium, 6.5% to 15% nickel, 2.5% to about5.0% copper, and 0.1% to 0.5% material selected from the groupconsisting of sulfur and selenium, up to .5% phosphorus, and remainderiron.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,961,777 Palmer June 5, 1934 2,009,713 Palmer July 30, 19352,523,000 Defranoux Sept. 19, 1950 FOREIGN PATENTS Number Country Date437,592 Great Britain Oct. 23, 1935 OTHER REFERENCES Alloys of Iron andChromium, vol. II, High Chromium, pages 431, and 432. Edited by Kinzelet al. Published in 1940 by the McGraw-Hill Book Co.

1. CHROMIUM-NICKEL STAINLESS STEELS DISPLAYING FREEMACHINING QUALITITES,TOGETHER WITH REDUCED TOOL ABRASION AND FREE-CHIPPING QUALITIESCOMPRISING ABOUT 18% CHROMIUN, 8% NICKEL, 3% COPPER, A MATERIAL SELECTED